Rapid detection of imminent failure in optical thermal processing of a substrate

ABSTRACT

A system for thermal processing of a substrate includes a source of radiation, optics disposed between the source and the substrate to receive light from the source of radiation at the optics proximate end, and a housing holding the optics and having a void inside the housing isolated from light emitted from the source. A light detector is disposed within the void in the housing to detect light from the optics emitted into the housing and send a deterioration signal. The system further includes a power supply for the source of radiation, and a controller to control the power supply based on the deterioration signal from the light detector.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/185,454 filed Jul. 20, 2005 entitled RAPID DETECTION OF IMMINENTFAILURE IN LASER THERMAL PROCESSING OF A SUBSTRATE by Bruce Adams, etal., which claims the benefit of U.S. Provisional Application No.60/627,529 filed Nov. 12, 2004, both of which are assigned to thepresent assignee.

This application contains subject matter related to U.S. Pat. No.7,129,440 issued Oct. 31, 2006 entitled SINGLE AXIS LIGHT PIPE FORHOMOGENIZING SLOW AXIS OF ILLUMINATION BASED ON LASER DIODES by BruceAdams, et al.; U.S. Pat. No. 7,135,392 issued Nov. 13, 2006 entitledTHERMAL FLUX LASER ANNEALING FOR ION IMPLANTATION OF SEMICONDUCTOR P-NJUNCTIONS by Bruce Adams, et al.; U.S. application Ser. No. 11/195,380filed Aug. 2, 2005 (U.S. Patent Application Publication No. 2006/0102607published May 18, 2006) entitled MULTIPLE BAND PASS FILTERING FORPYROMETRY IN LASER BASED ANNEALING SYSTEMS by Bruce Adams et al.; andU.S. patent application Ser. No. 11/198,660 filed Aug. 5, 2005 (U.S.Patent Application Publication No. 2006/0105585 published May 18, 2006)entitled AUTOFOCUS FOR HIGH POWER LASER DIODE BASED ANNEALING SYSTEM byDean Jennings, et al., all of which applications are assigned to thepresent assignee.

FIELD

The invention relates generally to thermal processing of semiconductorsubstrates. In particular, the invention relates to laser thermalprocessing of semiconductor substrates.

BACKGROUND

Thermal processing is required in the fabrication of silicon and othersemiconductor integrated circuits formed in silicon wafers or othersubstrates such as glass panels for displays. The required temperaturesmay range from relatively low temperatures of less than 250° C. togreater than 1000°, 1200°, or even 1400° C. and may be used for avariety of processes such as dopant implant annealing, crystallization,oxidation, nitridation, silicidation, and chemical vapor deposition aswell as others.

For the very shallow circuit features required for advanced integratedcircuits, it is greatly desired to reduce the total thermal budget inachieving the required thermal processing. The thermal budget may beconsidered as the total time at high temperatures necessary to achievethe desired processing temperature. The time that the wafer needs tostay at the highest temperature can be very short.

Rapid thermal processing (RTP) uses radiant lamps which can be veryquickly turned on and off to heat only the wafer and not the rest of thechamber. Pulsed laser annealing using very short (about 20 ns) laserpulses is effective at heating only the surface layer and not theunderlying wafer, thus allowing very short ramp up and ramp down rates.

A more recently developed approach in various forms, sometimes calledthermal flux laser annealing or dynamic surface annealing (DSA), isdescribed by Jennings et al. in PCT/2003/00196966 based upon U.S. Pat.No. 6,987,240, issued Jan. 17, 2006 and incorporated herein by referencein its entirety. Markle describes a different form in U.S. Pat. No.6,531,681 and Talwar yet a further version in U.S. Pat. No. 6,747,245.

The Jennings and Markle versions use CW diode lasers to produce veryintense beams of light that strike the wafer as a thin long line ofradiation. The line is then scanned over the surface of the wafer in adirection perpendicular to the long dimension of the line beam.

SUMMARY

A system for thermal processing of a substrate includes a source ofradiation, optics disposed between the source and the substrate toreceive light from the source of radiation at the optics proximate end,and a housing holding the optics and having a void inside the housingisolated from light emitted from the source. A sealant is between theoptics proximate end and the source of radiation. A light detector isdisposed within said void in the housing to detect light from the opticsemitted into the housing and send a deterioration signal. The systemfurther includes a power supply for the source of radiation, and acontroller to control the power supply based on the deterioration signalfrom the light detector.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an orthographic representation of a thermal flux laserannealing apparatus employed in the present invention.

FIGS. 2 and 3 are orthographic views from different perspectives ofoptical components of the apparatus of FIG. 1.

FIG. 4 is an end plan view of a portion of a semiconductor laser arrayin the apparatus of FIG. 1.

FIG. 5 is an orthographic view of a homogenizing light pipe for theapparatus of FIG. 1.

FIG. 6 is a schematic diagram of an optical system in accordance with anembodiment of the invention.

DETAILED DESCRIPTION

One embodiment of the apparatus described in the above-referencedapplication by Jennings et al. is illustrated in the schematicorthographic representation of FIG. 1. A gantry structure 10 fortwo-dimensional scanning includes a pair of fixed parallel rails 12, 14.Two parallel gantry beams 16, 18 are fixed together a set distance apartand supported on the fixed rails 12, 14 and are controlled by anunillustrated motor and drive mechanism to slide on rollers or ballbearings together along the fixed rails 12, 14. A beam source 20 isslidably supported on the gantry beams 16, 18, and may be suspendedbelow the beams 16, 18 which are controlled by unillustrated motors anddrive mechanisms to slide along them. A silicon wafer 22 or othersubstrate is stationarily supported below the gantry structure 10. Thebeam source 20 includes a laser light source and optics to produce adownwardly directed fan-shaped beam 24 that strikes the wafer 22 as aline beam 26 extending generally parallel to the fixed rails 12, 14, inwhat is conveniently called the slow direction. Although not illustratedhere, the gantry structure further includes a Z-axis stage for movingthe laser light source and optics in a direction generally parallel tothe fan-shaped beam 24 to thereby controllably vary the distance betweenthe beam source 20 and the wafer 22 and thus control the focusing of theline beam 26 on the wafer 22. Exemplary dimensions of the line beam 26include a length of 1 cm and a width of 66 microns with an exemplarypower density of 220 kW/cm². Alternatively, the beam source andassociated optics may be stationary while the wafer is supported on astage which scans it in two dimensions.

In typical operation, the gantry beams 16, 18 are set at a particularposition along the fixed rails 12, 14 and the beam source 20 is moved ata uniform speed along the gantry beams 16, 18 to scan the line beam 26perpendicularly to its long dimension in a direction conveniently calledthe fast direction. The line beam 26 is thereby scanned from one side ofthe wafer 22 to the other to irradiate a 1 cm swath of the wafer 22. Theline beam 26 is narrow enough and the scanning speed in the fastdirection fast enough that a particular area of the wafer is onlymomentarily exposed to the optical radiation of the line beam 26 but theintensity at the peak of the line beam is enough to heat the surfaceregion to very high temperatures. However, the deeper portions of thewafer 22 are not significantly heated and further act as a heat sink toquickly cool the surface region. Once the fast scan has been completed,the gantry beams 16, 18 are moved along the fixed rails 12, 14 to a newposition such that the line beam 26 is moved along its long dimensionextending along the slow axis. The fast scanning is then performed toirradiate a neighboring swath of the wafer 22. The alternating fast andslow scanning are repeated, perhaps in a serpentine path of the beamsource 20, until the entire wafer 22 has been thermally processed.

The optics beam source 20 includes an array of lasers. An example isorthographically illustrated in FIGS. 2 and 3, in which laser radiationat about 810 nm is produced in an optical system 30 from two laser barstacks 32, one of which is illustrated in end plan view in FIG. 4. Eachlaser bar stack 32 includes 14 parallel bars 34, generally correspondingto a vertical p-n junction in a GaAs semiconductor structure, extendinglaterally about 1 cm and separated by about 0.9 mm. Typically, watercooling layers are disposed between the bars 34. In each bar 34 areformed 49 emitters 36, each constituting a separate GaAs laser emittingrespective beams having different divergence angles in orthogonaldirections. The illustrated bars 34 are positioned with their longdimension extending over multiple emitters 36 and aligned along the slowaxis and their short dimension corresponding to the less than 1-micronp-n depletion layer aligned along the fast axis. The small source sizealong the fast axis allows effective collimation along the fast axis.The divergence angle is large along the fast axis and relatively smallalong the slow axis.

Returning to FIGS. 2 and 3 two arrays of cylindrical lenslets 40 arepositioned along the laser bars 34 to collimate the laser light in anarrow beam along the fast axis. They may be bonded with adhesive on thelaser stacks 32 and aligned with the bars 34 to extend over the emittingareas 36.

The optics beam source 20 can further include conventional opticalelements. Such conventional optical elements can include an interleaverand a polarization multiplexer, although the selection by the skilledworker of such elements is not limited to such an example. In theexample of FIGS. 2 and 3, the two sets of beams from the two bar stacks32 are input to an interleaver 42, which has a multiple beam splittertype of structure and having specified coatings on two internal diagonalfaces, e.g., reflective parallel bands, to selectively reflect andtransmit light. Such interleavers are commercially available fromResearch Electro Optics (REO). In the interleaver 42, patterned metallicreflector bands are formed in angled surfaces for each set of beams fromthe two bar stacks 32 such that beams from bars 34 on one side of thestack 32 are alternatively reflected or transmitted and therebyinterleaved with beams from bars 34 on the other side of the stack 32which undergo corresponding selective transmission/reflection, therebyfilling in the otherwise spaced radiation profile from the separatedemitters 36.

A first set of interleaved beams is passed through a quarter-wave plate48 to rotate its polarization relative to that of the second set ofinterleaved beams. Both sets of interleaved beams are input to apolarization multiplexer (PMUX) 52 having a structure of a doublepolarization beam splitter. Such a PMUX is commercially available fromResearch Electro Optics. First and second diagonal interface layers 54,56 cause the two sets of interleaved beams to be reflected along acommon axis from their front faces. The first interface 54 is typicallyimplemented as a dielectric interference filter designed as a hardreflector (HR) while the second interface 56 is implemented as adielectric interference filter designed as a polarization beam splitter(PBS) at the laser wavelength. As a result, the first set of interleavedbeams reflected from the first interface layer 54 strikes the back ofthe second interface layer 56. Because of the polarization rotationintroduced by the quarter-wave plate 48, the first set of interleavedbeams passes through the second interface layer 56. The intensity of asource beam 58 output by the PMUX 52 is doubled from that of the eitherof the two sets of interleaved beams.

Although shown separated in the drawings, the interleaver 42, thequarter-wave plate 48, and the PMUX 52 and its interfaces 54, 56, aswell as additional filters that may be attached to input and outputfaces are typically joined together by a plastic encapsulant, such as aUV curable epoxy, to provide a rigid optical system. An importantinterface is the plastic bonding of the lenslets 40 to the laser stacks32, on which they must be aligned to the bars 34. The source beam 58 ispassed through a set of cylindrical lenses 62, 64, 66 to focus thesource beam 58 along the slow axis.

A one-dimensional light pipe 70 homogenizes the source beam along theslow axis. The source beam, focused by the cylindrical lenses 62, 64,66, enters the light pipe 70 with a finite convergence angle along theslow axis but substantially collimated along the fast axis. The lightpipe 70, more clearly illustrated in the orthographic view of FIG. 5,acts as a beam homogenizer to reduce the beam structure along the slowaxis introduced by the multiple emitters 36 in the bar stack 32 spacedapart on the slow axis. The light pipe 70 may be implemented as arectangular slab 72 of optical glass having a sufficiently high index ofrefraction to produce total internal reflection. It has a shortdimension along the slow axis and a longer dimension along the fastaxis. The slab 72 extends a substantial distance along an axis 74 of thesource beam 58 converging along the slow axis on an input face 76. Thesource beam 58 is internally reflected several times from the top andbottom surfaces of the slab 72, thereby removing much of the texturingalong the slow axis and homogenizing the beam along the slow axis whenit exits on an output face 78. The source beam 58, however, is alreadywell collimated along the fast axis (by the cylindrical lensets 40) andthe slab 72 is wide enough that the source beam 58 is not internallyreflected on the side surfaces of the slab 72 but maintains itscollimation along the fast axis. The light pipe 70 may be tapered alongits axial direction to control the entrance and exit apertures and beamconvergence and divergence. The one-dimensional light pipe canalternatively be implemented as two parallel reflective surfacescorresponding generally to the upper and lower faces of the slab 72 withthe source beam passing between them.

The source beam output by the light pipe 70 is generally uniform. Asfurther illustrated in the schematic view of FIG. 6, further anamorphiclens set or optics 80, 82 expands the output beam in the slow axis andincludes a generally spherical lens to project the desired line beam 26on the wafer 22. The anamorphic optics 80, 82 shape the source beam intwo dimensions to produce a narrow line beam of limited length. In thedirection of the fast axis, the output optics have an infinite conjugatefor the source at the output of the light pipe (although systems may bedesigned with a finite source conjugate) and a finite conjugate at theimage plane of the wafer 22 while, in the direction of the slow axis,the output optics has a finite conjugate at the source at the output ofthe light pipe 70 and a finite conjugate at the image plane. Further, inthe direction of the slow axis, the nonuniform radiation from themultiple laser diodes of the laser bars is homogenized by the light pipe70. The ability of the light pipe 70 to homogenize strongly depends onthe number of times the light is reflected traversing the light pipe 70.This number is determined by the length of the light pipe 70, thedirection of the taper if any, the size of the entrance and exitapertures as well as the launch angle into the light pipe 70. Furtheranamorphic optics focus the source beam into the line beam of desireddimensions on the surface of the wafer 22.

One problem in laser radiation thermal processing is maintaining theintegrity of the optics and rapidly detecting its deterioration thuspreventing imminent failure of the laser source. To a large extent, theintegrity of the optics depends on of the condition of the interfaces atwhich the optics components are joined together. Typically, the opticscomponents are attached to each other at their interfaces withadhesives. If one of the components or the adhesive degrades,significant amounts of radiation power is scattered within the housingencapsulating the optics instead of propagating through the opticstoward the substrate. It is desired to restrict the damage to the onesection of components in which the failure is occurring. For example,the lenslets are epoxied to the laser bar stacks and may delaminate fromthe stacks causing the laser light to scatter within the chamber. It isdesired to restrict the damage to the lenslets and not allow thescattered radiation to heat up and degrade the other components, forexample, the PMUX and interleaver and interfaces attached to them.Conventionally, a thermocouple is used to measure an increase in theambient temperature within the housing or the temperature of opticalassemblies resulting from the increase of laser radiation. However, theresponse time of a thermocouple is often too slow to report a rapidcatastrophic system failure, and the system may collapse before anappreciable rise in the housing temperature occurs. A rapid indicationof the level of radiation energy within the housing is therefore desiredto detect a component failure and prevent the catastrophic deteriorationof the system. One aspect of this invention uses photodiodes to detectthe failure in the system components and enable a timely shutdown of thesystem.

Referring to FIGS. 3 and 6, the optics, including the interleaver 42,the quarter-wave plate 48, and the PMUX 52 and its interfaces 54, 56,are joined together and encapsulated inside a lightproof housing 68 ofthe beam source 20 of FIG. 1 having an output aperture 79 facing thesubstrate 22. Similarly, the lenslets 40 are bonded to the laser barsstacks 32 with an adhesive. A light detector or photo diode 81 and acontroller 90 are used to effectuate a fast intervention into operationof the laser thermal processing system by interrupting power from apower supply 100 to the laser bar stacks 32 upon deterioration of one ormore optical components in the housing 68 to prevent a catastrophicfailure of the thermal processing system.

The light detector 81 is preferably located inside the housing 68adjacent to the laser stacks 32, the interleaver 42, or the PMUX 52 andcan be supported by a support structure 82, such as a support ringattached to the housing 68. The light detector 81 should be out of thedirect path of the laser light assuming the optical components have notdegraded. It should also be out of the path of waste light, for examplewaste beams 110 from the interleaver 42 that are imperfectly reflectedand interleaved or the residual light 111 that is transmitted throughwavelength or polarization selective reflectors, such as the interfaces54, 56 in the PMUX 52. The photodetector 81 should also be out of thedirect path of the laser light reflected from the wafer 22, perhaps atangles that are not specular because of the wafer surface structure. Thephoto detector 81 should also not point to the radiation dumps used tosuppress the waste radiation. Instead, the light detector 81 should bepointing in a direction that is nominally dark. For example, the photodetector 81 may be located in back of the laser stacks 32 and be pointedalong an optical axis 112 directed to a portion of the housing 68 at thelateral side of the laser stack 32 that normally does not receiveradiation from the laser bars. When one of the optical components beginsto fail or its adhesive or encapsulant loosens, the tightly controlledoptical focusing is lost, and laser radiation from the bar stacks 32begins to propagate along unintended paths and strike unintendedreflective structures within the housing 68. That is, imminent failureis marked by an increase of ambient radiation within the housing 68 atthe laser wavelength.

Optionally, the support structure 82 for the photo detector 81 may be atranslation mechanism to move the light detector 81 vertically in orderto sense light radiation level in various areas across the housing 68.

The translation mechanism 82 can be fixedly attached to the frame of thehousing 68 and can be capable of extending and detracting along a pathacross the housing 68 to obtain the most optically advantageous positionfor the light detector 81. The translation mechanism 82 can include ahorizontal actuator that can move the light detector laterally in orderto adjust the distance between the light detector 81 and optics in thehousing 68. Optionally, a rotary actuator can be connected to thehousing 68 in order to rotate the light detector 81 around the optics inthe housing.

Referring to FIGS. 3, 4, and 6, the light detector 81 may be a photodiode to measure the light radiated within the housing 68 receivedwithin some field of view about the optical axis 112. A lens 114 may beused to control the field of view. An optical filter 116 may be disposedin front of the input to the light detector 81 to preferentially passthe laser wavelength and suppress thermal radiation at longerwavelengths. A comparator 86 receives the electrical output of the photodetector (such as, for example, the photocurrent from the photodiode) tocompare the measurements taken by the photo detector 81 with the baseamount of the scattered light radiated into the housing 68 under thenormal system operation. The light detector 81 is preferably sensitiveto the wavelength of the laser radiation, for example, 810 nm for GaAslaser bars. A silicon photodiode or pin detector provides the requiredsensitivity. A possible position for the light detector 81 is in back ofthe laser stacks 32 with a field of view pointing to the lateral side oraway from the laser stacks 32.

The photo diode preferably is a silicon photodiode made of “n” typesilicon material. The basic elements of photo diode 81 include a “p”layer formed on the front surface of the device. The interface betweenthe “p” layer and the “n” silicon is known as a pn junction.Alternatively, the photodiode 81 is formed of a “n” layer on a “p”substrate. Other types of silicon photodetectors are known, such as pinphotodiodes and charged coupled devices and photodetectors of othermaterials are available. Metal contacts are connected to the anode andcathode of the photo diode is the anode. Unillustrated biasing circuitryprovides the requisite biasing voltage to the photodiode and amplifiesand separates the photocurrent to the comparator 86.

The photodetector 81 may be disposed inside the housing 68 oralternatively disposed outside of it with either an opticallytransparent window or with an optical fiber receiving radiation insidethe housing 68 and conveying it to the photodetector located outside.Either zero or reverse bias photodiodes 74 can be used, although thereverse biased photodiodes are more preferable for the rapid detectionof the increased laser radiation in the housing 68 because theircircuits are more sensitive to light.

High voltage is preferably applied to the diode contacts of the reversebiased diode to increase the sensitivity of the diode to the radiation.The voltage is applied across the high resistance of the reversed biasedsemiconductor junction. The high resistance is reduced when light of anappropriate frequency impinges on the diode. For a fast response timerequired in the system 30, the resistance and operating voltage of thephoto diode 81 must be chosen corresponding to the operating wavelengthsbetween 810 nm and 1550 nm. Alternatively stated, the detected photonsgenerate electron-hole pairs in the vicinity of the pn junction, whichis detected in the sensing circuit as a photocurrent.

Referring to FIG. 6, the comparator 86 is an interface device thatreceives, processes and analyzes signals from the photo diode 81. Thecomparator 86 may include analog processing circuitry (not shown) fornormalization or amplification of the signals from the photo diode 81and an analog to digital converter (not shown) for conversion analogsignals to digital signals. The comparator 86 continuously monitors andtakes measurements of actual current signals from the photo diode 81 andcompares the processed signals from the photo diode 81 with apredetermined value corresponding to a baseline amount of scatteredlight in the housing during normal operation of the optical system 30.If the value of the signal from the photo diode 81 does not exceed thethreshold of the predetermined baseline value, the comparator does notact. Whenever or if the voltage level exceeds a predetermined referencelevel, the comparator 86 will generate a signal to the controller 90indicating the presence of a meaningful increase in radiating powerscattered within the housing 68. The comparator may also includeelectronic filtering to remove noise from the photocurrent so thatimminent failure is flagged only when the photocurrent exceeds athreshold for a predetermined length of time. Upon receipt of thisdetection signal, the controller 90 sends a control signal to the powersupply 100 to cease delivering power for the emission of the radiationbeam from the beam source 20. The comparator 86 can be located inside ofthe housing 68 apart from the photo diode 81, or it can be coupled andmoved with the photo diode 81 by the translation mechanism 82 or yetfurther alternatively may be stationary and linked to the moving beamsource 20 by flexible wiring. Alternatively, the comparator 86 may beincluded in the controller 90 positioned outside the housing 68.

The operations of the system of FIG. 1 can be coordinated by thecontroller 90. Controller 90 may be a general purpose programmabledigital computer connected to the comparator 86, the power supply 100,and, optionally, to the translation mechanism 82 supporting thephotodetector 81 as well as the translation mechanisms of the gantrystructure 10. The controller 90 can be programmed to stationarilyposition the photo diode 81 for the best exposure of the opticallytransparent window, to activate or interrupt the power supply 100, todetermine the level of the light radiation in the housing 68 and toactivate visual and sound emergency alarms, and, optionally, move thephoto diode 81 within the housing 68.

In operation, the power supply 100 provides electric power to energizethe beam source 20, which includes laser light source and optical system30, for emission of a downwardly directed beam 24 for thermal processingof the substrate. The light scattered in the housing 68 from the opticalsystem 30 under the normal operational conditions of the system 30 canbe sensed by the photo diode 81. The photo diode 81, which is sensitiveto the normal operational light energy emitted in the housing 68 by theinterleaver 42 and the PMUX 52, can generate a continuous currentresponse signal to the comparator 86 proportional to the intensity ofthe amount of the scattered light received. When one or more of theoptical components of the system 30, including the reflective andanti-reflective coatings, or adhesives which seal the interfaces of theinterleaver 42 and the PMUX 52 or the cylindrical lens 62, 64, 66, beginto fail, the light radiated through the ruptured component escapes fromthe optical system 30 into the housing 68. A large increase in radiatedpower scattered within the housing 68 is immediately detected by thephoto diode 81 and a current signal proportional to the increasedradiation is sent to the comparator 86. The controller 90 receives fromthe comparator 86 a signal associated with the condition that exceeds apredetermined baseline value corresponding to the radiation in thehousing 68 under the normal operations. This indicates the presence ofthe deterioration of the optical system 30, and the controller 90immediately disengages the power supply 100 based on the data from thecomparator 86 to stop further emission of the light radiation from thebeam source 20.

Although the invention has been described with respect to scanning of alinear laser beam, the invention may be applied to other thermalprocessing system involving high intensity radiation, for example, apixel pulsed laser system or a blanket irradiation system.

The system for optical control of thermal processing of a substrate hasseveral advantages. The system provides a measurement technique of thelevel of light radiation that is suitable for the high temperature andhigh radiation level environment of a laser thermal processing system.Furthermore, the light detector 81 can move within the housing 68 andconsequently the light detector 81 can be capable of obtaining the mostadvantageous position for the photo diode window or optical fiberconnection 92 to sense an excessive light radiation. The system can besimple, robust and inexpensive and does not require change to the layoutof the laser thermal processing system. Most importantly, the systemenables a much more rapid response than any existing measuring techniquefor the detection and prevention of catastrophic failure that wouldoccur in the absence of rapid intervention.

It may be possible to carry out the invention without either theinterleaver 42 or the polarization multiplexer 52 or without both ofthem. While the invention has been described in detail by specificreference to preferred embodiments, it is understood that variations andmodifications thereof may be made without departing from the true spiritand scope of the invention.

What is claimed is:
 1. A system for thermal processing of a workpiece,comprising: a source of electromagnetic radiation for thermal processingof the workpiece; optics comprising an assembly of plural opticalcomponents; a housing comprising plural walls enveloping said optics,said plural optical components corresponding to respective optical pathswithin said housing; a void inside the housing disposed away from saidrespective optical paths; a detector arranged to detect anelectromagnetic radiation level in said void and send an output signal;a power supply to provide power to the source of electromagneticradiation; and a controller to control the power supply based on theoutput signal from the detector.
 2. The system of claim 1 furthercomprising a sealant joining together at least one of said opticalcomponents with one of: (a) said housing, (b) said source, (c) anotherone of said optical components, wherein the sealant is a UV curingepoxy.
 3. A system for thermal processing of a workpiece, comprising: asource of electromagnetic radiation for thermal processing of theworkpiece; optics disposed between the source and the workpiece, saidoptics comprising an assembly of plural optical components, said opticsproviding an optical path between said source and said workpiece; ahousing comprising plural walls enveloping said optics, and a voidinside the housing away from said optical path, said void receiving nodirect radiation from said source and being illuminated upondeterioration of said optics; a detector arranged to detect anelectromagnetic radiation level in said void in the housing and acomparator responsive to said detector for producing a deteriorationsignal; a power supply coupled to the source of radiation; and acontroller to control the power supply based on the deterioration signalfrom the comparator.
 4. The system of claim 3 wherein said comparator isconfigured to produce the deterioration signal whenever an output ofsaid detector exceeds a predetermined threshold level.
 5. The system ofclaim 3 wherein said detector is positioned to detect an ambient levelof light electromagnetic radiation inside said housing.
 6. The system ofclaim 3 wherein said optics comprises an assembly of plural opticalcomponents, and a sealant joining together at least one of said opticalcomponents with one of: (a) said housing, (b) said source, (c) anotherone of said optical components, wherein the sealant is a UV curingepoxy; wherein said deterioration comprises deterioration of saidsealant.
 7. An apparatus for thermal processing of a workpiece,comprising: a source of electromagnetic radiation; an optical systemdisposed between the source and the workpiece; a housing comprisingplural walls enveloping the optical system therein; an electromagneticradiation detector disposed to sense a radiation level within a zone ofsaid housing that is outside of optical paths of said optical system,and a comparator coupled to said electromagnetic radiation detector andconfigured to produce a deterioration signal upon an output of saidelectromagnetic radiation detector exceeding a predetermined referencelevel; a power supply coupled to the source of electromagneticradiation; and a controller configured to interrupt power from the powersupply in response to the deterioration signal.
 8. The apparatus ofclaim 7 wherein said comparator is located outside of said housing. 9.The apparatus of claim 7 further comprising a sealant attaching saidoptical system to said housing.
 10. The apparatus of claim 1 whereinsaid source is inside said housing whereby said walls envelope saidsource and said optics.
 11. The apparatus of claim 3 wherein said sourceis inside said housing whereby said walls envelope said source and saidoptics.
 12. The apparatus of claim 7 wherein said source is inside saidhousing whereby said walls envelope said source and said optical system.13. The apparatus of claim 1 wherein said detector is located insidesaid housing.
 14. The apparatus of claim 13 wherein said detector islocated inside said void.
 15. The apparatus of claim 3 wherein saiddetector is located inside said housing.
 16. The apparatus of claim 15wherein said detector is located inside said void.
 17. The apparatus ofclaim 7 wherein said zone receives no direct electromagnetic radiationfrom said source and is illuminated only upon deterioration of saidoptical system.